SAW RAC devices (i.e., dispersive delay lines) have important uses in pulse compression radars, Fourier transform signal processing, etc. Other device configurations have been proposed over the years for achieving the requisite dispersion characteristics, whereby electromagnetic wave and ultrasonic delay lines were used to provide a given dispersive delay (i.e., a delay that is a desired function of frequency). More recently, SAW dispersive devices comprising ion-milled grating reflectors have been widely used. Unfortunately, the ion-milling of the grove reflectors requires an extra operation which is rather costly and time consuming. Moreover, ion-milled reflection SAW devices suffer from high insertion loss which, of course, affects efficiency. If the ion-milled grating reflector grooves are made deeper in an effort to make the gratings more effective in order to reduce insertion loss, the device then suffers from bulk-mode conversion that contributes to additional insertion loss and spurious responses.
A new reflector concept for SAW resonators was set forth in the article "New Types of SAW Reflectors and Resonators Consisting of Reflecting Elements With Positive and Negative Coefficients" by M. Takeuchi et al, IEEE Transactions on Untrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-33, No. 4, July 1986, pp. 369-374. The described resonator reflector has the advantages of large reflectivity, wide stopband, and small mode conversion into bulk waves.